1. Field of the Invention
The present invention generally relates to electric lamps, and more particularly, to electric lamps having a diffuse reflective coating and a silica coating for protecting the diffuse reflective coating.
2. Description of Related Art
It is known to use coatings on lamps which reflect various portions of the spectrum of light emitted from the lamp. Heat-resistant diffuse reflective coatings containing a refractory metal oxide such as zirconia, alumina, titania, etc., with a glass frit binder or a binder such as a mixture of boric acid and silica, have been used on the ends of arc tubes of high intensity discharge lamps, such as metal halide arc discharge lamps. See, for example, U.S. Pat. No. 3,374,377, the disclosure of which is expressly incorporated herein in its entirety. The reflective coatings reflect both visible and infrared radiation to maintain a relatively high temperature at ends of the arc tube and prevent ionized metal halides in the arc tube from condensing on otherwise relatively cool ends of the arc tube.
Heat-resistant diffuse reflective coatings comprising boron nitride have also been used on lamps, such as incandescent linear quartz heat lamps. See, for example, U.S. Pat. No. 5,168,193, the disclosure of which is expressly incorporated herein in its entirety. The reflective coating is applied to a portion of the outer surface of the lamp envelope for reflecting both visible and infrared radiation emitted by the filament. Applying the reflective coating on half of the linear surface of the envelope maximizes the radiant energy emitted in the direction of an object to be heated, while minimizing the radiant energy emitted in the opposite direction.
The diffuse reflective coatings or powder coatings are not very durable and typically have a relatively low abrasion resistance. Therefore, lamps having exposed coatings require careful shipping and handling. The diffuse reflective coatings which are water based, such as the boron nitride coatings, are also easily washed off. This is particularly a problem in industrial settings having the potential for condensation to build up when the lamp is cool. The condensation can build up and gradually wash off the diffuse reflective coating. Additionally, in many industrial settings the lamps are cleaned with acetone which washes off the diffuse reflective coating. Accordingly, there is a need for a protective overcoat for diffuse reflective coatings that increases abrasion resistance, increases moisture and acetone resistance, does not degrade the performance of the lamp, is relatively inexpensive to apply, and will withstand the high temperature conditions and thermal cycling experienced on incandescent and arc discharge lamps.